1. Field of the Invention
The present invention relates to an automatic control apparatus which controls an operating signal to be supplied to a control apparatus in such a manner that status variables of the control apparatus, such as temperature, flow rate, pressure, and level, coincide with target values.
2. Description of the Related Art
For control of this type, a PID (Proportional-Integral-Derivative) control apparatus is widely used due to its good general-purpose design. The conventional PID control apparatus performs a PID operation on the deviation between the operating signal of the control apparatus and a control signal output from the control apparatus used to acquire the operating signal of the control apparatus.
FIG. 1 shows a basic block diagram of the conventional PID control apparatus. A physical target value s is input to a reference input element 2, which converts the received physical target value s into a control target value x.sub.s. A deviation between the control target value x.sub.s and a control signal x of an object element 4, .epsilon..sub.s (=x.sub.s -x), is supplied to a PID controller 6. A transfer function Gc(S) of the PID controller 6 is expressed as follows. EQU Gc(S)=KP+KI/S+KD.S (1)
where
KP: proportional gain PA1 KI: integral time PA1 KD: differentiation time PA1 S: Laplace operator
An operating signal y (=.epsilon..sub.s.Gc(S)) output from the PID controller 6 is supplied to the object element 4. The object element 4 is also supplied with a disturbance g.
In this conventional PID control apparatus, the values of the individual parameters KP, KI, and KD are determined on the assumption that the object element 4 (the relation between the operating signal and the control signal) has a linear characteristic. In other words, the values of the individual parameters are determined by linearly approximating the characteristic of the object element in the vicinity of the operating signal y when a target value x.sub.s is given. In the conventional control apparatus, once the values of parameters are determined, they will not be set again even if the target value is changed. The conventional control apparatus cannot therefore provide satisfactory control results for object elements having a non-linear characteristic, such as an induction motor, a distillation plant, and a flow valve.
This problem results from the fact that the information source for a control arithmetic operation depends only on the deviation .epsilon..sub.s between the control signal x and the control target value x.sub.s at that time. In the differentiation operation, information regarding the correlation between the control target value x.sub.s and the control signal x will be lost. That is, the differentiation of the deviation in the conventional PID control is expressed as follows. EQU d.epsilon..sub.s /dt=dx.sub.s /dt-dx/dt (2)
Since the control target value x.sub.s is often constant, dx.sub.s /dt=0 so that the information regarding the correlation between x.sub.s and x, particularly, the information of the control operation concerning the magnitude of x with x.sub.s taken as a reference, will be lost as a consequence of the differentiation. In the case where an object element having a non linear characteristic is to be controlled, the values of parameters cannot be varied with a change in the target value, so that the original purpose of the differentiation to provide a control operation cannot be accomplished.
All the conventional PID control apparatuses have this shortcoming irrespective of whether they employ an analog system (continuous control system) or a digital system (sampled value control system).